1. Field of the Invention
The present invention relates to a method for manufacturing an electrode plate group for a prismatic battery, and more particularly to a method for manufacturing an electrode plate group having little variation in weight at low cost and with high productivity.
2. Description of Related Art
In a prismatic battery, an electrode plate group is formed by stacking rectangular-shaped positive electrode plates and negative electrode plates with separators interposed therebetween. This electrode plate group is accommodated in a prismatic battery case along with an electrolyte, and an opening at the top of the battery case is closed by a lid having a safety vent.
Electrode plates such as positive electrode plates and negative electrode plates need to be mass-produced since a number of electrode plates are used in each cell. If such electrode plates were to be manufactured individually, this would result in extremely poor productivity. Accordingly, single electrode plates are normally produced by manufacturing a large plate from which a plurality of electrode plates are cut and then simultaneously cutting out a number of single electrode plates from the large plate.
When single electrode plates are manufactured from a large plate, however, it is extremely difficult to avoid variations in the distribution of active materials between different positions on the large plate. As a result, when a plurality of single electrode plates are simultaneously cut from a large plate and the electrode plates are stacked without adjusting the positional arrangement of the electrode plates, the amount of active materials in the sets of plates formed by the stacked electrode plates will differ depending on where the single electrode plates were positioned on the large plate. When plates in such sets of plates are stacked alternately with single electrode plates of the opposite polarity to form an electrode plate group, there can be large variations in the weight of the positive electrodes or negative electrodes in the resulting electrode plate groups depending on where the single electrode plates in the sets of electrode plates were positioned on the large plate. As one example, when an electrode plate group is formed by stacking the heaviest positive electrode plates, the lightest negative electrode plates and separators, there will be a marked imbalance in capacity between the positive and negative electrodes. This leads to poor absorption of gas during charging, resulting in a rise in internal pressure and a risk of the battery leaking. Alternatively, this may lead to the safety vent opening frequently, resulting in a reduction in the cycle life of the battery due to a decrease in the amount of electrolyte, etc.
Japanese Laid-Open Patent Publication No. Hei. 11-154531 teaches one example of a method for manufacturing electrode plate groups with a similar capacity so as to overcome the above problem. In the disclosed method, the weights of single electrode plates are measured and the single electrode plates are then sorted according to their weight, with the electrode plates being combined so that the total weight of the single electrode plates composing an electrode plate group is within a predetermined ranges.
Alternatively, after electrode plate groups have been used to produce batteries, it is also possible to sort the batteries according to their capacity and to provide users with groups of batteries that are appropriate for the intended use of the batteries.
However, in order to sort the single electrode plates by weight, a single electrode plate weighing/sorting device for weighing the single electrode plates and then sorting them by weight needs to be set up on a single electrode plate production line. Accordingly, this method has a drawback of increased equipment costs.
In the other case where the electrode plate groups are made into batteries that are then sorted according to capacity, a capacity sorting device is required. This increases equipment costs and there is the further problem of the time required to measure the capacities of batteries, which lowers productivity.